Fuel cell system for reacting hydrocarbons

ABSTRACT

A fuel cell system of the high temperature type includes one or more fuel cells operating with an electrolyte such as an oxygen ion conductive solid or a carbonate melt to which fresh reaction gases containing hydrocarbon fuel and oxygen respectively are fed after preheating in heat exchangers. The system may also include a separate reformer for converting the hydrocarbons into hydrogen and other gases. A portion of the exhaust gases from the fuel side of the cell is recycled through the fuel cell by entrainment with fresh fuel gas by means of the aspirating effect of a nozzle through which fresh fuel gas is injected into the cell and the remainder of the exhaust gas from the fuel side together with oxygen impoverished gas discharged from the cell are led to an after-combustion catalyst in the form of a tubular structure surrounding the fuel cells and also the reformer component if one is utilized for burning of the remaining hydrocarbon content of the fuel gas. The heat generated during the after-burning phase in the catalytic zone is mostly transferred over a very short path to the fuel cells to maintain the fuel cells at their proper operating temperature and to supply the power requirements of the reformer if the latter is utilized. The remaining heat in the exhaust gases is conducted to the exchangers for heat transfer to the incoming fresh reaction gases.

United States Patent 1 1 Fischer et al.

[ 51 Feb. 27, 1973 FUEL CELL SYSTEM FOR REACTING HYDROCARBONS [75]Inventors: Wilfried Fischer, Neckargemund; Franz-Josef Rohr,Ober-Absteinach, both of Germany [73] Assignee: AktiengesellschaftBrown, Boveri 8:

Cie, Baden, Switzerland 22 Filed: Feb. 22, 1971 21 Appl. No.: 117,381

3,436,271 4/1969 Cole ..l36/86 E 3,516,807 6/1970 West et al......l36/86 R 3,577,329 5/1971 Shalit ..136/86 C Primary Examiner-HelenM. McCarthy Assistant Examiner-H. A. Feeley Attorney Pierce, Scheffler &Parker [57] ABSTRACT A fuel cell system of the high temperature typeincludes one or more fuel cells operating with an electrolyte such as anoxygen ion conductive solid or a carbonate melt to which fresh reactiongases containing hydrocarbon fuel and oxygen respectively are fed afterpreheating in heat exchangers. The system may also include a separatereformer for converting the hydrocarbons into hydrogen and other gases.A portion of the exhaust gases from the fuel side of the cell isrecycled through the fuel cell by entrainment with fresh fuel gas bymeans of the aspirating effect of a nozzle through which fresh fuel gasis injected into the cell and the remainder of the exhaust gas from thefuel side together with oxygen impoverished gas discharged from the cellare led to an after-combustion catalyst in the form of a tubularstructure surrounding the fuel cells and also the reformer component ifone is utilized for burning of the remaining hydrocarbon content of thefuel gas. The heat generated during the after-burning phase in thecatalytic zone is mostly transferred over a very short path to the fuelcells to maintain the fuel cells at their proper operating temperatureand to supply the power requirements of the reformer if the latter isutilized. The remaining heat in the exhaust gases is conducted to theexchangers for heat transfer to the incoming fresh reaction gases.

6 Claims, 2 Drawing Figures Zn, Kb

FUEL CELL SYSTEM FOR REACTING HYDROCARBONS The present invention relatesto an improved fuel cell system for conversion of the combustionenthalpy of hydrocarbons into electrical energy. The system includes oneor more relatively high-temperature fuel cells utilizing an electrolyteeither in the form of an oxygen-ion-conductive solid or a carbonatemelt, and a heat exchanger for effecting preheating of the entering,relatively cold, reaction gases such as air and propane by heatabsorption from relatively hot exhaust gases, and also in some cases areformer for converting the hydrocarbons in the gaseous fuel to hydrogenand other gases prior to entering the fuel cells themselves.

It is known that hydrocarbons can be re-formed above a temperature of500C, i.e. they can be converted with steam and/or carbon dioxide intohydrogen and carbon monoxide. In addition, fuel cells have already beensuggested in which reformed hydrocarbons are converted into electricalenergy. In this connection, reference is made to an article by H. Binderet al., in Electrochem. Acta 8, 781, (1963). Reformation of thehydrocarbons can be effected either in a catalyst containing reactorarranged in the flow path ahead of the fuel cells or on the fuel cellanode.

Since reformation of the hydrocarbons takes place at temperatures above500C, the fuel cells must be operated, because of the heat balance, atthe same or even a higher temperature. In the fuel cells, heat isreleased; in reformation of the hydrocarbons, heat is absorbed. For thisreason, fuel cells with an oxygenion-conductive solid electrolyte, orthose with a carbonate melt as an electrolyte are utilized. Theoperating temperatures of such fuel cells lie between 500 and 1,200C.See, for example, Swiss patents Nos. 429,858 and 446,456.

If hydrocarbons are to be reacted efficiently and essentiallymaintenance-free in a fuel cell system, the gasand heat currents must beso conducted that optimum operating parameters can be set and maintainedat the desired load output of the system. To this end:

1. It should be possible to start the system cold and to operate itcontinuously without the need for extraneous auxiliary equipment andwithout moving parts as much as possible.

2. the heat balance should be positive at each load level.

3. the operating temperature should be regulable.

4. formation of soot within the cell and reformer components of thesystem due to decomposition of the hydrocarbons should be avoided.

5. the fuel should be consumed as completely as possible.

In a known high temperature fuel cell system, e.g. as disclosed inGerman Patent Application P 15 71 9865-45, the reaction gases areconducted through a heat exchanger. Starting of the system is effectedby flame-heating, and formation of soot is prevented by connecting pipesbetween the fueland exhaust lines in the heat exchanger, which has theeffect that not only fuel gas, but also a fuel-exhaust gas mixture, isfed to the fuel cell or cells. For this reason, the aforesaidrequirements (1 to 5) can be met in principle. However, starting of thesystem by flame heating is te'chnically unsatisfactory. The exhaust gas(to prevent formation of soot) is added to the fuel gas in the heatexchanger, hence at a point where the exhaust gas is already partiallycooled. This has an adverse effect on the heat balance. Finally, thefuel content in the exhaust gas current issuing from the installation(it is unavoidable that a part of the fuel is not consumedelectrochemically) is lost for the maintenance of the heat balance.

A pipe constriction, or a nozzle, to carry along a second gas isdescribed, e.g. in the Newsletter of General Instrument Corporation,June, 1969, G1. Deliveries W TEMAR Boy Power Source.

In a fuel cell installation described by S. Baker (Baker et al., HighTemperature Natural Gas Fuel Cells, 1965), in Fuel Cell Systems,Advances in Chemistry Series 47 (1965), p. 247, in which a carbonatemelt serves as the electrolyte within the cells, the hydrocarbon is fedtogether with steam to the fuel cell-battery by way of a reformer. Theheat content-0f the exhaust gases is utilized to cover the heatrequirement of the endothermic reformer-reaction. The exhaust gasescontaining C0 are fed to the cathode to cover the CO consumption due tothe cathode reaction:

Because the exhaust gascontaining H 0 and CO is mixed with air, theaddition of H 0 and CO to the fuel necessary to prevent formation ofsoot cannot be covered directly from the same exhaust gas current.Rather the water required for the reformation process can only berecovered from the exhaust gas mixture mixed with air by separating itfrom the air by condensation.

The related further evaporation and heating then requires so much energythat the heat content either cannot, or at best can only be maintainedwith a very low efficiency.

Furthermore, it is known from other fields of application to utilizecatalysts for after-combustion of the remaining fuel not reactedelectrochemically in the cells. In this connection, reference is made toan article by E. Hermann, Apparatus and Plants For CatalyticAfter-Combustion Chem. Ing. Technik 37, pages 905 and 912, I965. Thecombustion of a fuel gas is here effected on a catalyst wall to whichthe fuel gas and the air are fed from different sides. But in theabove-mentioned Newsletter of General Instrument Corp. hydrocarbon andair are fed to the catalyst from the same side.

Summarizing the above-mentioned five requirements, the principal objectof the present invention is to utilize the portion of'the combustionheat content of the fuel gas not converted into electrical energy asfully as possible for maintaining the operating temperature and toeffect the supply and mixing of the reaction gases with the use of asfew auxiliary devices as possible such as pumps, etc., and thus torealize an essentially maintenance-free and easy-to-start system. Thisdesired objective is made practical according to the invention in atechnically advantageous manner, starting from the aforementioned knowntechniques, primarily by a combination of the following measures:

The fuel gas is conducted after passing through the heat exchanger andin advance of the fuel cell battery,

as well as in advance of a reformer, if one is provided, through anozzle over which passes a portion of the exhaust gas from the fuel sideof the fuel cell battery. The jet of fresh fuel issuing from the nozzlethus functions as a jet pump or aspirator entraining with it thesurrounding exhaust gas so as to recycle this portion of the exhaust gasthrough the fuel cells and a separate reformer also, if provided, alongwith fresh fuel thus to avoid formation of soot within the cells andreformer without preceding cooling and without the necessity forproviding a separate auxiliary pump for this purpose. The part of theanode exhaust gas not re-cycled through the fuel cell is fed to anafter-combustion catalyst. The air now impoverished in O, is likewisesupplied to the catalyst from the exhaust side of the fuel cell. Thiscan be effected on the same, or opposite side of a porous catalyst wallto which the fuel exhaust gas is conducted. In this manner,after-combustion of the fuel portion which was not reactedelectrochemically within the cells is made possible. The combustionheat, in addition to the heat generated internally in the fuel cellbattery due to an operating efficiency of less than 1, serves tomaintain the operating temperature of the fuel cell battery, to coverthe heat requirement of the endothermic reformation reaction and to heatthe battery from its initial cold state for starting. In addition, theoperating temperature of the fuel cell battery can be easily regulatedin this manner since the after-combustion heat (with the same electricpower) is increased, or reduced, as the case may be by adding more orless fuel.

The improved fuel cell battery structure in accordance with theinvention will now be described in more detail with reference to twoembodiments thereof and which are illustrated in the accompanyingdrawings wherein:

FIG. 1 is a view in longitudinal section through a completely housedfuel cell battery and utilizes a ceramic oxygen-ion-conductive solidelectrolyte; and

FIG. 2 is also a view in longitudinal section through a slightlydifferent fuel cell battery in which all operating components are housedbut utilizing as an electrolyte for the fuel cell system a carbonatemelt and also a separate reformer located in the gas flow path inadvance of the fuel cells.

With reference now to FIG. 1, the fuel cell battery is of the typewherein an electrolyte in the form of a ceramic oxygen-ion-conductivesolid is utilized, and wherein re-formation of the hydrocarbon takesplace at the anodes of the fuel cells. Fuel gas, e.g. propane is fedthrough a tube 1 from a source of supply, not illustrated, into andthrough a heat exchanger 2a where it is pre-heated by the final exhaustgases from the battery and thence through a nozzle 3 into the first of aplurality of the solid electrolyte fuel cell modules 4 arranged inseries. After leaving the first cell, the fuel gas enters and flowsthrough the second cell, the fuel gas being partly consumedelectrochemically as it flows through the cells. The mixture ofcombustion gases and unreacted fuel flows out of the last cell 4 in theseries in exit pipe 5, and a portion of this exhaust gas mixture is, inaccordance with the invention, recycled through the fuel cells. Moreparticularly, a portion of the gas mixture flowing through pipe 5 isdelivered into an annular duct arrangement 30 surrounding the nozzle 3where it is drawn, i.e. aspirated, into the fuel cell 4 along with freshfuel gas discharged from nozzle 3, in accordance with the jet pumpprinciple. Nozzle 3 is so dimensioned that the quantity of combustiongas re-cycled into the fuel cells is greater than the propane, actuallyabout three times as much. In this manner, formation of soot within thecells is avoided. The remainder of the exhaust gas mixture, i.e. theportion not re-entering the fuel cells, flows out of the exit pipe 6into the upper part 7 of one chamber 26 provided in housing 24 forreceiving the fuel cells 4. A tubular porous catalytic afterburnerstructure 10 surrounding the fuel cells reaches to the bottom wall ofhousing 24 which establishes the lower boundary wall for chamber 26, andthe tubular after-burner 10 itself also defines a boundary wall forchamber 26 and through which the exhaust gas mixture from the fuel cellsis caused to flow.

Air to furnish oxygen for the fuel cells is delivered by pump P intopipe 8 from whence it flows into and through heat exchanger 2b where itis also preheated by the final exhaust gases from the battery. The airthen flows into and through the fuel cell modules 4 where its oxygencontent is partly consumed and is then exhausted from pipe 9 at the exitfrom the last cell in the series into chamber part 7. As indicated inthe drawing, the surface area for the heat exchanger 2b for the enteringair is seen to be much greater than that of the heat exchanger 20 forthe entering propane. Assuming that for the illustrated embodiment twiceas much oxygen is supplied as is consumed electrochemically within thefuel cells, the surface area of heat exchanger 2b will be ten timesgreater than that of heat exchanger 2a, this for the reason that theamounts of heat to be supplied are in a ratio of about 10 1.

From the chamber part 7, the gaseous mixture therein of combustion gas,un-reacted fuel, oxygen and nitrogen flows into the space containing thefuel cells 4 and thence laterally outward through the porous catalysttube 10 of the afterburner-catalyzer into a chamber 11 formed betweencatalyst tube 10 and the side wall of housing 24, this chamber beingseparated from the fuel cell chamber 26 by a ring-shaped end closurewall 24a at the upper end of tube 10. The remainder of the fuel gas isburned in the catalyzer tube 10 which surrounds the fuel cells 4 and theresulting heat flowing over a short path to the fuel cell helps, tomaintain the operating temperature of the fuel cells at about 800C. Heatproduced by the catalyzer 10 is also used for starting up of the fuelcells from their cold state.

When starting the battery in a cold state, the gases are at first flowedthrough the cells 4 unused to the catalyst tube 10. Catalysticcombustion is possible, starting from a temperature of about 200C, i.e.it suffices to bring one point of the catalyst tube 10 to thistemperature by an electric heater in the form of a coiled hot wirefilament 22, and then this points heats itself and the surroundingneighborhood. With rising temperature, more and more of the gases arereacted electrochemically in the fuel cells. A temperature regulator 23is employed to shut off the supply of current to filament 22 at anominal reference temperature value above 200C. Temperature regulator 23and hot-wire filament 22 are integrated into the housing 24, but it isalso possible to join up the ignition device by means of a plugconnection.

The exhaust gas after transfer of heat to the fuel cells 4 and flowingfrom the catalyzer into chamber 11, and no longer containing anyun-reacted fuel, is discharged from pipe 11a into chamber another 25 inhousing 24 which houses the heat exchangers 2a, 2b and gives up most ofits remaining heat content to the latter. Final discharge of the cooledexhaust gases is from chamber 25 to the atmosphere through a pipe 12.

In the embodiment illustrated in FIG. 2, the fuel cells are of the typewherein a carbonate melt is utilized as the electrolyte. Re-formation ofthe hydrocarbon is effected in a separate reformer component arranged inthe gas flow path in advance of the fuel cells 4'.

The hydrocarbon fuel gas is conducted under pres sure from a source ofsupply through a tube 1' into and through heat exchangers 2a in the samemanner as in the embodiment of FIG. 1. However, prior to entering thefuel cells located in chamber 26, the fuel gas is fed through a nozzle3' to the reformer tubes of a reformer component 13 filled with catalystmaterial. After leaving reformer 13, also located in chamber 26', thefuel gas is passed through the one or more fuel cells 4' and dischargesfrom the latter through pipe 5'. In the interest'of simplicity, only onefuel cell 4 is depicted in FIG. 2. However, several such cells may beutilized in the manner of FIG. 1. A part of the exhaust gases issuingfrom pipe 5' enter a duct 3a surrounding nozzle 3' and are re-cycledthrough reformer 13, and hence also through the fuel cells 4', by thejet pump action of nozzle 3'. These exhaust gases contain H 0 and CO andthe addition of these serve to prevent deposit of soot within thereformer 13 as well as in the fuel cells 4. The fuel-exhaust mixtureentering the fuel cells, heated to the temperature of the reformer, isreacted at least in part electrochemically upon power consumption.

The portion of the exhaust gases from the fuel cells 4 that is notre-cycled through reformer 13 is discharged through pipe 6 into chamber7.

Air to furnish oxygen for the fuel cells is supplied from pump P througha pipe and nozzle 14 into and through heat exchanger 2b for pre-heatingand thence into and through the fuel cells 4'. A portion of the oxygencontent in the air is consumed in the cells and the remainingcomposition discharged from the fuel cells is passed through pipe 27into chamber 11 by-passing the catalyst tube 10' through which flowsfrom the inner side of the catalyst tube, from chamber 26', the exhaustgas mixture on the fuel side. The remaining fuel is consumed in thiscatalyst tube. As in the case of the embodiment of FIG. 1, heat producedby afterburning at the catalyst tube 10' is transferred over a shortpath to the fuel cells 4 to maintain the cells at their proper operatingtemperature and to also supply the power requirement of the reformer 13.All exhaust gases are then conducted out of chamber 11 through a tube 15into and through chamber 25' which houses the heat exchangers 2a and 2band are discharged through an outlet pipe having two branches 16, 17. Aportion of the exhaust gases flows directly to the atmosphere throughbranch pipe 17. The remainder of the exhaust gases, however, is leadthrough branch pipe 116 into and through a condenser in which a coolantis circulated through a coil 21. Water in the exhaust gases is removedby condenser 20 and the so treated exhaust gases are then conducted to amixing chamber 19 which includes a duct portion 19a enveloping fresh airnozzle 14 that serves to effect re-cycling of the dried exhaust gases bymeans of the jet pump action established by nozzle 14. The driedcombustion products added to the fresh air contain among others CO whichmust be present on the cathode side of the fuel cells 4' so that thecarbonate ions consumed on the anode side can be produced again on thecathode side. In this connection, reference is made to theabovementioned article by Baker et al.

Starting of the fuel cell system depicted in FIG. 2 is effected in amanner similar to that described in connection with the FIG. 1embodiment.

In conclusion, the improved fuel cell arrangements in accordance withthe inventive concept as herein described thus combine the followingadvantageous measures:

1. Heat exchange wherein heat from the hot exhaust gases is absorbed bythe entering reaction gases with optimum dimensioning of theirrespective heat exchange surface areas.

2. Mixing exhaust gases with fresh fuel entering the fuel cells and alsoa separate reformer, if provided, by jet pump action of a fuel gasnozzle within the battery to prevent formation of soot within the fuelcells and reformer.

3. Catalytic after-combustion of the remainder of the fuel gas to coverthe heat requirement of the fuel cells and of the heat exchanger.

4. Supply of CO, with fresh air to the cathode side of the fuel cells.

5 Starting of the fuel system with low power.

6. Closed transportable arrangement of the various components of thefuel cell system.

We claim:

I. A high temperature fuel cell battery which comprises a housingproviding therein a first chamber in which are located one or more fuelcells of the high temperature type operating with an electrolyte such asan oxygen ion conductive solid or a carbonate melt and to which freshreaction gases containing hydrocarbon fuel and oxygen respectively arefed, a tubular porous catalytic afterburner structure surrounding saidfuel cells, means for initially heating said afterburner structure,first and second heat exchangers located in a second chamber of saidhousing for preheating respectively the fresh fuel and oxygen containinggases in advance of being introduced into said fuel cells for reaction,fuel and oxygen containing gas conduits connecting the outlets from saidheat exchangers with said fuel cells, said fuel gas conduit including ajet pump located in said first chamber and actuated by the fresh fuelgas and including means for delivering a portion of the exhaust gasmixture from said fuel cells thereto so as to be entrained with freshfuel gas for recycling-through said fuel cells, said jet pump being sodimensioned that the quantity of combustion gas in the exhaust gasmixture is greater than the fresh fuel gas thereby to minimize formationof soot within the fuel cells, the remaining portion of the exhaust gasmixture from said fuel cells being delivered to the interior of saidtubular catalytic afterburner structure for flow in an outwardlydirection therethrough into a third chamber provided within saidhousing, the remaining hydrocarbon content in said exhaust gas mixturebeing burned in said afterburner structure and generating heat which istransferred inwardly therefrom to said fuel cells for maintaining saidcells at their proper operating temperature, and conduit means forconnecting said third chamber with said second chamber for deliveringthe burned gases to said first and second heat exchangers.

2. A high temperature fuel cell battery as defined in claim 1 whereinsaid means for initially heating said after-burner structure includes anelectrically heated filament located in heat transfer relationtherewith.

3. A high temperature fuel cell battery as defined in claim 2 and whichfurther includes a temperature regulator for effecting a shut-off of theelectrical current applied to said filament when said filament hasattained a nominal temperature characteristic of independentcontinuation of the after-combustion reaction.

4. A high temperature fuel battery as defined in claim 1 wherein acarbonate melt serves as the electrolyte for the fuel cells and whichfurther includes a mixing chamber in which fresh oxygen containing gasis mixed with gas exhausted from said heat exchangers prior to enteringsaid second heat exchanger.

5. A high temperature fuel cell battery as defined in claim 4 whereinone portion of the gas exhausted from said heat exchangers is dischargedto atmosphere and another portion thereof is led to said mixing chamberby way of a condenser to effect removal of its moisture content.

6. A high temperature fuel cell battery as defined in claim 1 and whichfurther includes a reformer through which fresh fuel gas combined with aportion of the exhaust gas mixture by the action of said jet pump is fedprior to being introduced into said fuel cells, said reformer being alsosurrounded by said tubular catalytic afterburner structure and whichreceives heat therefrom to furnish its power requirement.

2. A high temperature fuel cell battery as defined in claim 1 whereinsaid means for initially heating said after-burner structure includes anelectrically heated filament located in heat transfer relationtherewith.
 3. A high temperature fuel cell battery as defined in claim 2and which further includes a temperature regulator for effecting ashut-off of the electrical current applied to said filament when saidfilament has attained a nominal temperature characteristic ofindependent continuation of the after-combustion reaction.
 4. A hightemperature fuel battery as defined in claim 1 wherein a carbonate meltserves as the electrolyte for the fuel cells and which further includesa mixing chamber in which fresh oxygen containing gas is mixed with gasexhausted from said heat exchangers prior to entering said second heatexchanger.
 5. A high temperature fuel cell battery as defined in claim 4wherein one portion of the gas exhaUsted from said heat exchangers isdischarged to atmosphere and another portion thereof is led to saidmixing chamber by way of a condenser to effect removal of its moisturecontent.
 6. A high temperature fuel cell battery as defined in claim 1and which further includes a reformer through which fresh fuel gascombined with a portion of the exhaust gas mixture by the action of saidjet pump is fed prior to being introduced into said fuel cells, saidreformer being also surrounded by said tubular catalytic afterburnerstructure and which receives heat therefrom to furnish its powerrequirement.